Pre-selection of sex has been accomplished in many species of livestock following the development of safe and reliable methods of separating sperm cells (also referred to as “sperm”) into enriched X chromosome bearing and Y chromosome bearing populations. See for example methods and apparatus disclosed by WO 00/06193; WO 02/043574; WO 01/85913; WO 99/33956; WO 01/40765; WO 98/34094; WO 99/42810; WO 02/043486.
A significant problem with sex-selected sperm cells may be that separation of sperm cells at rates sufficient to produce sex-selected insemination samples or sex-selected inseminates which are viable or sufficiently fertile for commercial application by conventional technology has necessitated increasing fluid stream pressure of flow cytometers or flow sort instruments to about 50 pounds per square inch. With respect to sperm cells of many species of mammals entrained in fluid streams having flow characteristics resulting from this application of pressure, the viability, motility, or other fertility characteristics may be altered.
Another significant problem with sex-selected sperm cell inseminates or sex-selected sperm cell insemination samples can be the vast difference in sperm cell fertility characteristics which can vary greatly between samples. As such, success of artificial insemination performed under substantially identical conditions can result in correspondingly different pregnancy rates or cannot achieve the threshold of fertility in the context of conventional procedures for successful fertilization of eggs whether in-vitro or in-vivo.
Additionally, cryopreservation and thawing of sex-selected sperm can result in substantially reduced motility, percent intact acrosomes and post thaw survival time, all of which can militate against successful use of cryopreserved-thawed sex-selected sperm for the in-vitro or in-vivo fertilization of eggs of the same species of animal.
Additionally, certain animals, such as dairy cows, are known to have reduced fertility as compared to heifers. An adverse side effect of acquiring certain genetic traits advantageous to milk production can be concurrent with acquisition of genetic traits which lower fertility of dairy cows. Moreover, as to the example of dairy cows, it may also be desirable to increase the proportion of female offspring by the use of sex-selected sperm. In order to successfully breed animals having reduced fertility, especially in the instance of utilizing sex-selected sperm to increase the number of female offspring, it would be useful to have an inseminate, and a method of producing such inseminate, having increased fertility in order to increase pregnancy rates.
Therefore, a need exists for a method of increasing fertility of sperm within a sex-selected inseminate to overcome reduced fertility in inseminates whether due to low numbers of sperm contained in the inseminate, low fertility of sperm relating to animal to animal variation, the process of sex-selection, the low fertility of the female animal, or the like.
Additionally, a need exists for an assay from which fertility of sex-selected sperm cells can be compared directly in-vivo (for example, in conjunction with artificial insemination procedures) and in-vitro (for example, in conjunction with IVF procedures).
The instant invention addresses the need for a sex-selected inseminate having increased fertility to overcome reduced fertility due to limited numbers of sperm, low fertility of sperm relating to animal to animal variation or the process of sex-selection, and the low fertility of the female animal.